The importance of the littoral epiphyton as food for commercial fish in the recent African man-made lake, Nyumba ya Mungu reservoir, Tanzania

Abstract

Nyumba ya Mungu reservoir is a man-made lake in the Kilimanjaro region of northern Tanzania. The commercial fisheries are principally based on the cichlids, Sarotherodon pangani Lowe, S. jipe Lowe, S. esculentus Graham, and Tilapia rendalli Boulenger. Examination of gut contents revealed that the dominant species, S. pangani and S. jipe, fed on the epiphyton attached to the vegetation in the littoral zone, S. esculentus was a phytoplankton filter feeder, and T. rendalli consumed macrophytes and associated epiphytes. A fish exclosure quantitatively demonstrated that the epiphyton community was appreciably depleted by the grazing of fish. After 35 days the mean epiphyton density on the submerged surfaces of Cyperus articulatus L. within the exclosure was 285 times 103 individuals cm-2 shoot, but only 181 times 103 individuals cm-2 shoot on grazed surfaces outside the exclosure. The differences were statistically significant, and corresponded to a difference in biomass of about 6.5 g organic dry weight m-2 lake surface. Vertical profiles of periphyton density on Cyperus indicated that fish grazed mainly below a depth of 100 mm from the water surface. The littoral epiphyton commercial fish food chain, which may be unique, is considered to predominate in Nyumba ya Mungu, and is compared with the major food chains of commercial fish in some other African natural and man-made lakes.